Two-stroke versus four-stroke engines: how they work, pros and cons

Two-stroke versus four-stroke engines: how they work, pros and cons

Two-stroke versus four-stroke engines: how they work, pros and cons

My friends who have made the switch talk about differences in traction, engine braking, power delivery, sound and overall feel. So let's take a closer look to answer some basic questions, like what is the difference between a four-stroke engine and a two-stroke engine? How do four-stroke and two-stroke engines work? What are the pros and cons of each? And which one is best for you?

Four strokes versus two strokes: how they work

Few street bikes have two-stroke engines these days, so this is mostly a question for off-road riders. To keep things simple for this article, we'll stick to modern single-cylinder dirt bike engines in two- and four-stroke formats. Let's take a look at the similarities and differences.

Both two-strokes and four-strokes are internal combustion engines that use the energy created by burning a mixture of air and fuel to push a piston down. A connecting rod and crankshaft transfer the up and down motion of the piston into a rotary motion, which makes its way to the rear wheel of the motorcycle, pushing it forward.

When we say “stroke,” we are referring to the piston traveling up or down within the cylinder. In a four-stroke engine, the first stroke is intake, the second is compression, the third is combustion, and the fourth is exhaust. High school teachers trying to get the attention of hormonal teenagers sometimes use the more memorable terms suck, squeeze, pop, blow to describe the four strokes. Here's what happens with each shot.

Intake : The intake valve in the head opens as the piston moves downward, drawing in a charge of fuel. The engine is inhaling here.

Compression : The intake and exhaust valves are closed as the piston moves upward, compressing the fuel and air mixture.

Combustion : The intake and exhaust valves remain closed while the ignition system fires the spark plug, igniting the fuel-air mixture and pushing the piston back, providing power to the rear wheel.

Exhaust : The exhaust valve or valves open as the piston moves upward again, expelling the burned fuel and air mixture through the exhaust system. Then the cycle begins again.

Note that all of this is happening above the top of the piston. This is why four-stroke cylinder heads are much larger than two-stroke cylinder heads. The head must house the camshafts, valves, intake and exhaust ducts and the spark plug. Beneath all this, the crankshaft sits in an oil bath inside the crankcase. As the crank turns, it slings oil onto the cylinder walls, crank and rod bearings to keep everything sufficiently lubricated. Most motorcycles also have pressurized oil systems that spray oil where it is needed for lubrication and cooling.

A two-stroke engine follows the same suck-squish-bang-blow steps, but combines the intake and compression strokes, then combines the combustion and exhaust strokes, so the entire process is completed in two strokes instead of four . This means that a two-stroke engine produces power every time the piston moves down, while a four-stroke engine produces power every time the piston moves down. That's how. 

Two-strokes use the crankcase as part of the intake tract, using the volume above and below the piston. This being the case, the crankshaft cannot sit in an oil bath like a four-stroke. Instead, two-strokes have oil mixed with the fuel, and that residual oil lubricates the cylinder walls, crank, and rod bearings. 

Additionally, the cylinder head of a two-stroke engine is basically just a cap on top of the cylinder used to hold the spark plug. There are no valves or openings in the cylinder head. Instead, there are openings in the cylinder, called ports, that direct the flow of the intake charge and are covered and uncovered as the piston rises and falls in the cylinder. If you think of valves as inlet and outlet ports for the fuel-air mixture and exhaust, the piston in a two-stroke acts as a vertically sliding door to open and close the ports. The magic of a two-stroke is how it uses the motion of the piston, as well as the space above and below the piston, to accomplish the suck-squish-bang-blow sequence. That's how. 

Intake and compression : As the piston moves upward, a vacuum is created beneath the piston, drawing the fuel-air mixture into the crankcase. The compression stroke also occurs simultaneously above the piston as it travels upward. There is a reed valve within the intake tract between the crankcase and the fuel source in a two-stroke engine. It is a vacuum-actuated one-way valve that allows the two-stroke to draw fuel and air through it as the piston moves upward, but closes as the piston moves downward, preventing the fuel/air mixture from traveling backward, away from the combustion chamber.

Combustion and exhaust : The spark plug ignites the fuel-air mixture and pushes the piston downward, producing power, while the fuel-air mixture under the piston is forced through the transfer port from the bottom to the top of the engine. As the piston moves downward toward the bottom of its combustion stroke, it exposes the exhaust port in the cylinder wall. This is perfectly positioned to allow the pressure beneath the piston into the crankcase to expel the burned exhaust gases, simultaneously replacing them with a mixture of fuel and fresh, unburned air through the transfer ports.

You're probably thinking “OK, whatever, nerd! Both motors turn a wheel, so what the hell is wrong with going with one on top of the other?

The pros and cons of four-stroke and two-stroke engines

There are pros and cons to both types of engines, and they provide different driving experiences, which is why some of my friends have switched.

Four-stroke Pros : Four-strokes produce power only a quarter of the time compared to two-strokes, but a longer power stroke provides much more torque, so four-strokes produce better power at lower rpm and more linear and predictable thrust throughout the rev range. To complement the four strokes, four-strokes require a heavier flywheel to give the crank the inertia needed to keep it rotating between combustion strokes. The combination of less frequent power pulses to the rear wheel and the heavier flywheel gives four-strokes better traction in certain off-road conditions. Additionally, four-strokes typically offer much more engine braking, which many riders prefer. Notably, four-strokes do not have oil mixed with their gas, so they have cleaner exhaust emissions.

Four-stroke vs. Four-stroke : Historically, four-strokes were heavier than two-strokes, simply because there are more parts needed to run them, though the difference is fading as four-strokes become lighter. Heavier flywheels can cause a bike to handle differently and make it more resistant to changes in direction. Four-strokes are also more difficult to repair because they are more complex. This is a wash, in my book, because while two-strokes are much easier to rebuild, they need service more frequently.

Two-Stroke Pros : As mentioned above, two-strokes have historically been lighter and simpler. There are fewer moving parts to wear out, and high-end rebuilds can be done in a morning before an afternoon ride. Two-strokes tend to have mind-blowing power delivery as the rpm rises. Modern two-strokes are more controllable and perform better at low rpm thanks to improvements in the power valves, which are essentially small valves that can open or close the exhaust port size depending on engine speed.

Cons : Pre-mixing oil and gas is a minor hassle that's starting to wear thin due to the mess it creates. This scam will soon be history due to the popularity of TPI (transfer port injected) bikes. It's certainly a bit of a hassle to carry oil with me on long enduros to measure and add to my fuel tank at gas stops. This is strictly based on the one time my oil bottle leaked onto my snacks in my hydration pack.

Because two-strokes rely on sparser lubrication to keep running, their pistons, rings, cylinders, and cranks wear out more quickly, so you should plan on replacing parts more frequently on a two-stroke than on a four-stroke. The two-strokes also have a very unique exhaust design, with large, thin-walled pipes that are susceptible to dents and damage quite easily from off-road obstacles. Finally, two shots are dirty. Their exhaust is smoky and contains a fair amount of unburned gas, which is quite harmful to the environment. 

What's beautiful both in two and in four

Either way, your engine fires thousands of times per minute as you drive, combining perfect timing of ignition, fueling and scavenging to produce incredible power from a small, lightweight package. They are generating their own electricity to recharge their batteries and/or power their own ignition coils. They are turning the water pumps, ensuring everything stays at ideal operating temperatures via thermostats. I've been messing around with internal combustion engines since I was about five years old. I'm 41 now and still fascinated by how they work.

And the sound? If you've made it this far, you know that four-strokes go braaap and two-strokes go ring-a-ding-ding . Personally, I love the sound of two-strokes. The sound and smell make the hairs on my neck stand up and my eyes water. For me, there's no more addictive feeling than when a two-stroke is "on the pipe."

Four-strokes sound great too, except when it's a Double A hooking the limiter behind me on a hare run, which usually makes me pee a little, as it's very scary.

Either way, two-stroke or four-stroke, the bikes are fantastic. Ride what suits you best or just ride what you can get your hands on and have fun.

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